The operations of many floating vessels, such as semi-submersible drilling rigs, drill ships, and pipe-laying ships, are impeded by sea swell. Sea waves impart an up-and-down motion to a vessel, commonly referred to as “heave,” with the period of the waves ranging anywhere from a few seconds up to about 30 seconds or so and the amplitude of the waves ranges from a few centimeters or inches up to about 15 meters (about 50 feet) or more.
This up-and-down motion imparted to the vessel from the waves is then correspondingly imparted to any loads or structures attached to the vessel. In particular, this heave motion of the loads or structures extending from the vessel is often highly undesirable, and even dangerous, to equipment and personnel. For example, when attempting to drill a wellbore in the sea bed, the heave motion can cause a corresponding motion of the drill string. The up-and-down movement of a drill bit attached to the end of the drill string is highly undesirable and can severely restrict the operating window of the rig. For example, it has been estimated that in the North Sea, as much as 20% of rig operating time is lost “waiting on weather” when the sea would be calmer.
Heave compensation is directed to reducing the effect of this up-and-down motion on a load attached to the vessel. “Passive” heave compensation systems are typically used by fixing the load to a point, such as the sea bed. Sea swell may then cause the vessel to move relative to the load, in which a passive compensator uses compressed air to provide a low frequency dampening effect between the load and the vessel.
Further, “active” heave compensation systems may be used that typically involve measuring the movement of the vessel using a measuring device, such as a motion reference unit (“MRU”), and using a signal from the MRU that represents the motion of the vessel to compensate for the motion. The signal is used to control a drive, such as a drawworks, that moves a connection device, such as a traveling block or a crane hook, relative to the vessel. A drawworks may be used to control the connection device, in which the drawworks is a winch that is typically connected to the connection device by a cable that passes through a block and tackle arrangement. The drawworks can reel the cable in-and-out to cause the connection device to be raised and lowered relative to the vessel. The principle behind active heave compensation is to move the connection device in a manner equal to, but opposite, the heave motion of the vessel to cancel out the heave motion from being imparted to the load so that the desired motion of the load is achieved irrespective of the motion of the vessel.
Despite the advance in both passive and active heave compensation systems, however, heave compensation remains a priority to increase the safety and efficiency of drilling vessels.